In this sense it's a far better reason for a suit than for a band t-shirt. A suit is the primary output of a tailor, a band t-shirt is a commercial by-product of a band. There are better ways to support a band than buying a t-shirt - buying their records or seeing them live for instance. There is no better way to support a tailor.
As for the circular reasoning on its dirty - I agree but it's the same for both suit and band t-shirt. You have to have a reason to own it which I've given you.
Regardless of that owning and wearing a suit because I think it is a beautiful example of clothes design, regardless of whether it supports the creator or not, is a valid thing to do in itself that is neither narcissistic nor an example of social pressure.
And in case you think I'm desperately trying to come up with reasons but that they're not actually things any real person would do, I own only two suits, one of which (by Oswald Boateng) I own precisely for this reason - because I believe it to be a beautiful item (or more precisely two beautiful items) of clothing.
But the problem is still that your argument applies to lost of clothes, not just suits. Shirts are less practical than t-shirts. Buttons are fiddly and unnecessary, collars a pointless throw back to necktie and don't get me started on the practicality of cuffs. Most shoes are sub-optimal from a pure practicality point of view. Ironically many trainers aren't actually good for running because of the way they're balanced.
Or my other clothes were in the wash. Happy now?
Is that not a clear enough example of why your argument isn't just about suits when I can just reuse your answer an it works?